1. Field of the Invention
The present invention relates to an optical recording medium, and more particularly, to an optical recording medium which has a high reflectivity in the wavelength range of 630.about.790 nm, and which information can be reproduced from by a digital versatile disc player (DVDP) and a compact disc player (CDP).
2. Description of the Related Art
The recording area per recording unit of an optical recording medium is smaller than that of a conventional magnetic recording medium, so that the optical recording medium has been used as a high density recording medium. The optical recording medium is classified into read only memory (ROM) type which only reproduces already recorded information, write once read many (WORM) type which is recordable just once and readable many times, and an erasable type which is erasable and rewritable after recording. The information recorded in the optical recording medium should be reproduced by a player for the ROM type medium. To this end, the optical recording medium must satisfy specification of a conventional standardization (ORANGE BOOK), so that reflectivity of 65% or more and carrier-to-noise ratio (CNR) of 47 dB or more are required.
In a recordable optical recording medium, the recorded information can be reproduced based on a change in the reflectivity caused by a physical modification, phase change or change of magnetic properties in a recording layer before and after recording. Also, for making the optical recording medium compatible with a CD, a long-term data stability, a high recording sensitivity capable of supporting the high-speed recording rate, and economical advantages are required as well as the above mentioned high reflectivity and CNR characteristic. In order to improve the characteristics of the optical recording medium and make the manufacturing process easy, various optical recording media being formed of various materials have been suggested, and some of which are in practical use.
In response to demands for a high density optical recording medium, DVD has been developed, reproducing data by 650 nm diode laser and having 4.7 GB capacity. A DVD player can also play discs of CD format, but unfortunately cannot play discs of CD-R format, which are recordable and readable by CD equipment. For playback by DVD equipment, media should have more than 30% reflectivity at 650 nm. CD-R discs have less than 15% reflectivity at 650 nm due to high absorption of the recording layer at 600.about.750 nm, which prevents their use in DVD players.
As a conventional optical recording medium, Japanese Laid-open Patent Publication No. showha 63-268142 discloses a recording medium having a structure in which a sensitizing layer formed of gelatin, casein or polyvinyl alcohol (PVA), and a metal thin film formed of chromium (Cr), nickel (Ni) or gold (Au) are sequentially stacked on a substrate. According to the optical recording principle of the recording medium, the metal thin film absorbs heat of an irradiated laser beam, and thus the sensitizing layer and the metal thin film are deformed to form a recording pits. However, the recording pit of this recording medium are exposed so that it is difficult to store the recorded information on a long-term basis.
U.S. Pat. No. 4,983,440 discloses a recording medium having a structure in which two metal thin films as a recording layer, and a protecting layer for protecting the recording layer are sequentially stacked on a substrate. However, this recording medium has a very low reflectivity of 20% or less, so the recording medium is not compatible with conventional CDS.
According to U.S. Pat. No. 4,990,388, a recording medium has a light absorbing layer of a thickness of 20.about.500 nm, containing cyanine dye, a light reflecting layer formed of gold (Au), silver (Ag), copper (Cu), aluminum (Al) or alloys thereof, and a protecting layer are sequentially stacked on a substrate, and an intermediate layer having a thickness of 2 nm.about.500 .mu.m is formed between the substrate and the light absorbing layer. The surface layer (the intermediate layer acts as a surface layer when the intermediate layer is thick) existing between the substrate and the light absorbing layer is deformed by heat generated when the light absorbing layer absorbs laser beams to form a pit for optical recording.
U.S. Pat. No. 5,213,955 discloses an optical recording medium in which a light absorbing layer having a thickness of 250.about.410 nm, containing dye, a light reflecting layer formed of Au, Ag, Cu, Al or alloys thereof, and a protecting layer are sequentially stacked on a substrate. In the optical recording medium, the side of substrate contacting the light absorbing layer is deformed by the laser beam to achieve optical recording.
As described above, in the conventional optical recording medium, a recording layer generates heat by absorbing laser beams to deform the substrate, thereby forming a recording pit. To this end, the recording layer contains a dye as a light absorbing material. However, because such dye is expensive, the manufacturing costs increase if the optical recording medium contains a dye. Thus, making a dye containing layer thin has been attempted, however, the recording characteristics thereof have been unfavorable.
Also, U.S. Pat. No. 5,318,882 discloses an optical recording medium having an improved stability to light and heat, which adopts a recording layer which contains a light-vulnerable organic dye, such as a cyanine dye, and a light stabilizer for complementing the dye, such as nitrosodiphenylamine and derivatives thereof. However, the optical recording medium has poor reliability and may lower the recording sensitivity, and a 3T signal is easily damaged. Also, the optical recording medium cannot be compatible with a CD player and a DVD player.
Japanese Laid-open Patent Publication No. Heisei 3-215466 discloses a recording medium having a structure in which a recording layer and a metal reflecting layer are sequentially formed on a substrate, and the recording layer contains phthalocyanine as a near infrared ray absorbent. However, the wavelength of a reading laser beam is limited to 790.+-.20 nm, and a reflectivity at 650 nm is lower than a predetermined level, so the recording medium cannot be used for a DVD player.